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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

GYPSUM

157

density 1-1-1-3 or. after compression caused by 1000 shakes, about 1-8. The
chemical composition is moderately constant; the loss on ignition never reaches
5% and usually does not exceed 3% ; the alkalinity is somewhat less than in
limes, only 4-6 c.c. of N/io-acid being required per 0-5 gram of substance.
The essential components usually vary between the following limits :

Silica
Alumina .
Ferric oxide
Lime

19-26%
4-10
2-4

57-67

tl             d          CaO + MgO

U.lcl/LL 3, cUlU.    _ - , _ 

Si02 - (A1203 + Fe203

The hydraulic index usually lies between 0-42 and 0-50 but may reach 0-60,
and the Michaelis hydraulic modulus varies between 1-7 and 2-2, its mean value
being about 2. According to Le Chatelier, the proportion of lime should be
such that, when the components are expressed in chemical equivalents, the ratio

CaO + MgO      

 - __  . - _ -------        l

Si02 + A1203

As regards the accessory components, good Portland cement should not
contain more than 3% of MgO or 1-2% of SO3) or sulphides in appreciable quan-
tity. This cement requires 27-30% of water for gauging and usually sets in a
few hours.

The Official Italian Regulations prescribe for Portland cements : specific
gravity not less than 3-05 ; not more than 2 (or 20)% of residue to remain on a
sieve of 900 (4900) meshes per sq. cm. ; initial set of the normal paste to occur
later than i hour and final set between 5 and 12 hours ; after 28 days, the strength
of the normal mortar briquettes to be not less than 20 and 220 kilos respectively
per sq. cm. towards tension and compression.

Slag cements have an apparent density usually less than i and a specific gravity
2-6-2-8, the values for Grappier's cements being o-8-i-i and 2-8-3 ; mixed cements
approach one or the other class, according to their composition. All these
cements require 30-4.0% of water and set after some hours, almost always before
24 hours. As regards their chemical composition, slag cements contain calcium
sulphide in appreciable amount, which may reach or surpass 3-4%, and they
are usually richer in alumina and poorer in lime than Portland cements. Grap-
pier's cements are, however, poor in alumina and rich in silica, of which they
contain 22-30% ; the loss on ignition is generally less than with hydraulic
limes and mostly about 5%. Another characteristic of Grappier's and slag
cements is the fineness of grinding : they leave almost no residue on the 900-
mesh sieve and usually not more than 10 % on the 49oo-mesh.

Mention must be made of the so-called sand cements, which are used, especi-
ally in America, for street paving, dykes, aqueducts and canals, and have recently
been introduced into Italy. They consist of intimate mixtures of Portland
cement and siliceous rock (sand, arenaceous deposit, granite, etc.) which are
ground together and are moderately rich in silica and unattackable silicates
(42% or more).

Tables V, VI and VII contain examples of the compositions and properties
of hydraulic limes and cements.

GYPSUM

Ordinary gypsum is obtained by heating hydrated calcium sulphate at
a moderate temperature and is composed essentially of calcium sulphate
still containing a part (about one-fourth) of the water of crystallisation.
The principal impurities which may be present are clay, oxide of iron,
calcium carbonate, sand, and sometimes pyrites and bituminous matter.'iii> cements the Official [taliitu h't'^altilions prescribe ; a. specific